A display device including a light emitting element (hereinafter, referred to as a light emitting device) has a wider viewing angle and a higher response characteristic and operates with lower power consumption compared with a display device having a liquid crystal element, namely a liquid crystal display device. Therefore, the light emitting device has been actively developed.
A light emitting element includes an organic material or an inorganic material between a pair of electrodes. By applying current to the organic material or inorganic material and exciting a light emitting material, a predetermined luminescent color can be obtained. To increase emission luminance of the light emitting element, a large amount of current may be supplied, in other words, high voltage may be applied to the pair of electrodes; therefore, the advantage of low power consumption cannot be attained. In addition, deterioration of the light emitting element may be accelerated by applying a large amount of current.
Hence, a light emitting element in which emission luminance is increased by stacking a plurality of light emitting elements and applying current which has the same current density as a single layer is proposed (refer to Patent Document 1: Japanese Patent Laid-Open No. 2003-272860). By utilizing this light emitting element, a predetermined luminance can be obtained by a light emitting element having a stacked-layer structure even if current which has less than half current density of a single layer is applied. For example, it is said that n times luminance can be accomplished without increasing current density if n light emitting units having the same structure existing between electrodes are provided. At this time, it is mentioned that drive voltage also becomes n times or more; however, there is a great advantage that n times luminance can be obtained without sacrificing lifetime.